The present invention relates to a hydraulically operated continuously variable transmission having a hydraulic pump coupled to an input shaft and a hydraulic motor coupled to an output shaft, the hydraulic pump and the hydraulic motor being interconnected by a closed hydraulic circuit.
Hydraulically operated continuously variable transmissions are known in the art as disclosed in Japanese Laid-Open Patent Publication Nos. 57-76357 and 56-143857, for example.
In such hydraulically operated continuously variable transmissions, even when the inlet and outlet ports of a hydraulic pump are coupled to each other by a clutch valve, the hydraulic pump draws and discharges working oil in response to rotation of an input shaft. When the viscosity of the working oil is higher at low temperature than it is at normal temperature, the instantaneous load produced by the oil drawing and discharging action of the hydraulic pump upon rotation of the input shaft driven by an engine is large regardless of the clutch valve being actuated to interconnect the inlet and outlet ports of the hydraulic pump. Therefore, the engine cannot be started smoothly by relatively small initial power of a starter or the like.
In addition, since in a hydraulically hermetic chamber is filled with oil during engine shutdown, when the engine is started, resistance is produced by the plungers of the hydraulic pump which operate in and stirs the oil in the hydraulically hermetic chamber Therefore, the engine cannot be started smoothly by the starter. The resistance generated by the pump plungers when starting the engine is large at low temperature since the viscosity of the oil is high.